1. Field of Invention
The current invention relates generally to apparatus, systems and methods for monitoring electrical signals. More particularly, the apparatus, systems and methods relate to measuring an electrical current. Specifically, the apparatus, systems and methods provide for non-contact measurement an electrical current with magnetic materials.
2. Description of Related Art
Traditional current measuring devices include current transformers (CTs) that utilize a magnetic core as a component in the measuring circuit. The magnetic core shape is typically a cylindrical or rectangular prism with a central hole. The core is often constructed by stacking thin laminated magnetic grade steel forms to a prescribed height. A hole through the core perpendicular to the planes of the laminates is provided for electrical current carrying conductor(s) to pass through and thus form the primary winding of a two winding transformer. A second insulated copper conductor is wrapped through the central opening and around the core material for multiple turns thus making a secondary winding for the current transformer. An ammeter is used to measure the secondary current which is proportional to the primary current up to the point where the core is saturated with magnetic flux.
Versions of the current transformer with provisions for the core to open and to be placed around the primary current conductor(s) are known as split-core current transformers. A split-core CT that can be hand held and includes a mechanism to facilitate opening and closing of the core is known as a “clamp-on” ammeter. The movable parts of the core are called “jaws” and they made so the mating faces are flat and make intimate contact with each other when closed thus minimizing the air gap space. Since transformers in general and CTs in particular can only measure alternating current (AC), additional instrumentation is needed to measure direct current (DC) or other unidirectional currents. Magnetic transducers can be employed in split core and Clamp-on meters to add DC capability to iron core current transformers. The transducer normally used is a Hall-Effect device.
The Hall-Effect device can be configured as an integrated circuit implemented on a linear bipolar silicon chip. This version of the Hall-Effect device produces an output voltage linearly related to the amplitude and phase of a magnetic field normal to plates of a Hall device in the silicon chip. The voltage generated by the magnetic field is known as the Hall voltage.
When the magnetic flux in a core produced by a DC current in the linear region of the core iron passes through the Hall sensor, a voltage is developed that is an analog measure of the current magnitude. In clamp CTs, the Hall-Effect device is positioned on the face of one of the jaws such that the magnetic flux in the core passes perpendicularly through the silicone chip when the jaws are closed and an electrical current is flowing in the primary conductor(s). An amplifier is used to increase the Hall voltage to a level that can be read by a voltmeter. The voltmeter is calibrated with scaling factors such that the meter reading has the units of current but this is not a precise meter. In order to make accurate DC or AC measurements with Hall-Effect devices inserted in magnetic cores, magnetic flux nullification must be used to make the net flux in the core zero which is complex and expensive. In the aluminum refining industry, electrolysis currents of 400 kA can be measured to 0.1% accuracy with elaborate Hall-Effect DC current transducers. Thus, the use Hall-Effect devices in current measurements is well established but often very costly.
The magnetic iron in prior art current measuring devices is the source of errors and losses. Errors are caused by the residual magnetism, saturation of the magnetic material and non-linearity of the iron. Losses result from eddy currents induced in the core, hysteresis losses and copper loses in the secondary winding. Core and coil losses can be reduced with improved materials and efficient designs but they cannot be eliminated and there is the penalty of increased manufacturing costs. The application of iron core current transformers is primarily limited to 60 Hz symmetrical sine waves because DC components create economical and technical problems and high frequencies increase the inductance impedance to unacceptable levels.
In the field of protection of electrical power equipment such as transformers, generators, reactors, and other expensive apparatus, current transformers are one of the key components. These special purpose current transformers must provide information to the relays that determine if and when to operate the protective circuit breakers to prevent catastrophic damage to the electrical system. However, if the CT information is incomplete or inaccurate, then an unnecessary and costly false shut down may occur or worse, the fault may not be cleared. A vast amount of literature deals with the short coming of the current transformers and what has to be done to mitigate them.
The primary functional problem with the current transformer in protection systems is saturation of the iron core. Over currents can cause the core to saturate and become flat topped which is detrimental to the protective system but saturation from a DC component is a more serious problem because transformer action completely ceases and recovery is slow. During power system surges and faults, the current wave is usually asymmetrical and that is where the DC component arises.
Another issue with current transformers is the need to supply a burden because they have a current output. Therefore, current transformer secondary windings require a volt-ampere rating sufficient to meet the load demand plus the losses of the secondary leads that carry the current to that load. In some relaying applications, longer leads than are technically feasible are needed and that can cause the protection to be compromised even more.
Current transforms contribute to the fixed losses (eddy currents & hysteresis) as well as the variable losses (copper) of power systems. Reduction of these loses has become a priority in the effort to conserve resources and protect the environment.
There are additional issues with current transformers used to measure currents in conductors operating at high voltage (above 600 volts). The large physical size, weight, and power losses all lead to major compromises in the application of the iron core transformers for current measurements in high voltage current transformers (HVCT). Three designs are in use 1) current transformers placed in a metal container at the top of an insulating housing (“live tank” or top core), 2) current transformers placed at the bottom of an insulating housing (hairpin or “dead tank”) and 3) current transformer mounted part way down in the insulating housing (“eye-bolt” or cascade). In all three configurations, the designs are far from what is needed; therefore, a better current measuring device is needed.